This invention relates to an extrusion device for making thermoplastic articles molded from a heterogeneous blend of incompatible polymers. Especially preferred polymers are a polyolefin first polymer and a second polymer, incompatible with the polyolefin. The invention specifically relates to an extrusion device for a multiple parison die head.
U.S. Pat. No. 4,410,482 discloses the manufacture of thermoplastic articles from a heterogeneous blend of polyolefin first polymer and an incompatible second polymer. The articles produced according to that patent have the polymer present as a multitude of thin, substantially two-dimensional, parallel and overlapping layers in the polyolefin and the articles in the form of containers are disclosed to exhibit permeation barrier characteristics greatly increased in comparison with containers made from the polyolefin alone. Other patents which disclose articles having a similar construction include U.S. Pat. Nos. 4,444,817 and 4,416,942.
The aforementioned patents form the basis of what is known as the Dupont laminer flow technology. In the DuPont process the matrix resin and the barrier resin, such as Dupont's Selar RB, a modified nylon are dry mixed prior to being fed into the extrusion screw. A prerequisite to providing the desired permeability barrier is in controlled mixing of the barrier resin with the matrix resin. Proper mixing will produce many large, essentially two dimensional, barrier walls within the matrix resin. If the mixing is insufficient, the pellets of the barrier resin will not stretch to form the barrier walls, thus resulting in little improvement in the permeability resistance of the finished article over that provided by the matrix resin itself. If the barrier resin is over mixed, the barrier resin will break apart into small particles with little improvement in permeability resistance resulting.
The formation of desirable heterogeneous resin structure has been successfully accomplished with single parison die heads. However, attempts to produce barrier containers with multiple parison die heads, particularly three or more parison die heads, have had little success. In many cases, the resulting containers have no, or only little, improvement in permeability resistance. An investigation by the present inventor regarding flow patterns of a heterogeneous resin blend flowing in an extrusion blow molding machine has provided the following explanation of the difference in barrier performance of containers from single parison die heads compared to multiple parison die heads.
As the pellets of the barrier resin reach their melting temperature, streaks of the barrier resin will be formed. These streaks are formed by shear forces in the resin caused by the rotating screw and are substantially circumferential. As the resin flows from the extrusion screw and through the die head to form a tubular parison, the streaks form barrier walls in the matrix resin which, in the finished container, overlap one another to provide permeability resistance in the container wall. In the case of a single parison center-fed die head, the barrier resin platelets formed in the extrusion screw remain relatively undisturbed as the resin flows into the die head around the core to form a tubular parison. When the parison is blown, the platelets remain to form discontinuous, substantially two-dimensional overlapping thin layer barrier walls within the matrix resin to provide permeability resistance in the container wall. In a single parison side fed die head, although the flow patterns are less desirable in terms of maintaining the optimum resin structure, the deviation from the optimum resin structure is normally not large enough to cause a significant loss in the barrier performance.
When the resin is fed from the extrusion screw to a multiple parison die head, such as a triple parison die head, the division of the heterogeneous but well structured resin melt into three separate conduits in the transfer pipe disrupts the desirable structure of the barrier resin. When the resin is divided, many of the platelets are broken apart such that in the finished container, there are gaps or windows in the container wall having few or no barrier walls. These containers have no or only little barrier improvement over that provided by the matrix resin individually.
It is an object of the present invention to provide a multiple parison blow molding machine for producing containers of a matrix resin having thin layer barrier walls formed therein.
Applicant's co-pending patent application, Ser. No. 418,554, filed Oct. 10, 1989, provides a multiple parison die head which reconstructs a destroyed heterogeneous resin structure after the resin has been divided and fed to separate dies within the multiple parison die head. In contrast, it is a further object of the present invention to avoid the destruction of the heterogeneous resin structure altogether.
The extrusion device of the present invention consists of a uniquely configured screw extruder and adapter for feeding separate resin streams to a multiple parison die head. The screw extruder of this invention, like conventional extruders, includes a screw shaft and flight disposed within a barrel. The extrusion screw immediately upstream from the screw tip, does not contain any flights.
The barrel inner surface surrounding the shaft terminal portion is radially spaced from the shaft and includes ribs that project radially inwardly toward the shaft forming a close clearance with the shaft. These ribs extend longitudinally in the bore and are continued in the adapter along the screw tip. The ribs form a number of arcuate flow passages circumferentially about the shaft. The number of ribs and flow passages is equal to the number of parisons produced by the associated multiple parison die head with the resin from each flow passage being directed to a separate die.
The adapter is coupled to the end of the barrel and directs the resin from the arcuate flow passages into separate feed tubes. The adapter has a generally conical inlet into which the tip of the screw shaft projects. The inlet also has radially inwardly projecting ribs that form a smooth continuation of the ribs in the barrel. The adapter ribs and inlet are inclined inwardly in the direction of downstream resin flow generally parallel to the shaft tip. The inlet and ribs thus form a number of irregularly shaped funnel portions around the screw tip to direct the resin smoothly from the arcuate flow passages into the separate feed tubes.
The resin in the arcuate flow passages will be pushed longitudinally through the flow passages by resin upstream and still in contact with the screw flight. The rotation of the screw shaft will cause the resin to flow circumferentially, i.e., to circulate, within each arcuate flow passage. This circumferential and longitudinal flow of the resin will cause the soft pellets of the barrier resin to elongate and stretch to form platelets within the matrix resin. These platelets form the barrier walls in the finished plastic container. By first dividing the resin in the extruder barrel into separate streams, then stretching the barrier resin to form platelets, the laminar resin structure is formed and conveyed in a manner similar to a single parison die head. The structure is not later destroyed or damaged by dividing the resin stream.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.